Casimir IV, byname Casimir Jagiellonian, Polish Kazimierz Jagiellończyk (born Nov. 30, 1427—died June 7, 1492) grand duke of Lithuania (1440–92) and king of Poland (1447–92), who, by patient but tenacious policy, sought to preserve the political union between Poland and Lithuania and to recover the lost lands of old Poland. The great triumph of his reign was the final subjugation of the Teutonic Knights (1466).

Casimir was the second son of Władysław II Jagiełło and his fourth wife, Zofja Holszańska. His father was already over 75 at Casimir’s birth, and his brother Władysław III, three years his senior, was expected to become king before his majority. Casimir was thus the second in succession to the throne, and, after Władysław had succeeded his father in 1434, he became the legal heir. Strangely, little was done for his education; he was never taught Latin, nor was he trained for the responsibilities of office, despite the fact he was the only brother of the sovereign. Yet the necessity of taking office was thrust upon him in 1440, when the grand duke of Lithuania, Sigismund, was murdered. The boy was sent to Wilna to act as governor for his brother, but he was proclaimed grand duke in a coup d’etat by the leading boyars (nobles), who evidently hoped to use him as a convenient tool.

The coup practically severed the ties between Lithuania and Poland, but these were restored after Władysław III’s death in the Battle of Varna against the Turks (Nov. 10, 1444). The Poles, having to elect a new king, had no other candidate but Casimir. The young man, despite his lack of experience, knew how to wield his new power. He acted to preserve the hereditary rule of the dynasty in Lithuania with no connection with Poland other than the common monarchy, and, when he was finally crowned king of Poland (June 25, 1447), he had succeeded in affirming his right to live in Lithuania and choose his counsellors freely. Considering his deeds and policy (no personal utterances of his are recorded), it may be inferred that he regarded himself more as the head of a dynasty than as the elected king of Poland. His policy, therefore, was partly family policy, and in cases of conflict between dynasty and state the former had priority. His marriage to Elizabeth of Habsburg in 1454 had clear political aims; as the daughter of Albert II of Habsburg, Elizabeth had claims to Bohemia and Hungary. In fact, this first connection between the Habsburgs and the Jagiellons was a happy one; because of her six sons and seven daughters (born between 1456 and 1483), Elizabeth was called the “mother of Jagiellons.” Casimir did everything he could to provide his children with advantageous marriages. In this he was more than successful: his eldest son, Władysław, became king of Bohemia (1471) and of Hungary (1490); three others were his successors on the thrones of Lithuania and Poland; one became an archbishop and, later, a cardinal. Five of his daughters were married to German princes, as a result of which the Polish name Casimir became a familiar one among German dynasties. When he died he left a dynasty renowned among the courts of Europe.

In foreign policy, Casimir had few far-reaching plans or great ambitions. He neither organized a crusade against the Turks as his brother had done, nor did he build up an efficient defense system against the aggressions of the grand duchy of Moscow. He failed also to support Moscow’s enemies and contented himself with the favourable treaty of 1449, which, however, did little to prepare Lithuania for the attacks that were to begin in 1486. Thus, a number of Russian princes, vassals of Lithuania, went over to the Muscovite grand duke after 1486 because they had obtained no protection from Casimir.

Similarly, in Poland the king showed little initiative in foreign policy. When the Prussians, however, revolted in 1454 against their overlord, the Teutonic Order, and placed themselves under the protection of Casimir, he was aware that this was a unique opportunity to destroy the power of the order. In October 1453 the cities and gentry of Prussia, in a dispute with the order (which had been excommunicated by the pope and put under the ban of the Holy Roman Empire), placed themselves under Casimir’s overlordship. Subsequently, in February 1454, they renounced their allegiance to the order. They then captured 57 towns and castles, and on March 6, 1454, Casimir incorporated all of Prussia with Poland, with a guarantee of autonomy and of freedom from taxation. When, as a result, war broke out and Polish troops were severely defeated by the order near Konitz (Sept. 18, 1454), it was mainly Casimir’s perseverance and stubbornness that led eventually to success after a bloody victory at Puck (Sept. 17, 1462). The papacy finally intervened, and by the second Treaty of Toruń (Thorn; Oct. 19, 1466), all of western Prussia, called “Royal Prussia,” was ceded to Poland, while the remainder of Prussia was held by the Teutonic Order as a fief of the Polish crown. Though the order thus retained a part of its former territory and “Royal Prussia” was not formally incorporated but only united with the Polish kingdom while preserving its own diet and administration, this treaty was Casimir’s most important foreign policy success.

In domestic affairs Casimir was relatively passive but anxious to preserve the prerogatives of the crown, notably his right to nominate bishops. In the question of territories in dispute between his two states (Volhynia and Podolia) he favoured Lithuania. During the war against the Teutonic Order he was forced to grant the Polish nobility substantial concessions by the Privilege (statute) of Nieszawa (November 1454); these, however, became important only after his death, and royal power was not greatly diminished during his lifetime.

Casimir was neither a splendid ruler nor a good and wise administrator, but a mistrusting, cautious, and sober head of a large family who regarded Lithuania as his personal estate. His reign was remembered as being both successful and peaceful.

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(1427-92). As grand duke of Lithuania from 1440 to 1492 and king of Poland from 1447 to 1492, Casimir IV was neither a man of great ambition nor a great warrior. Yet during his reign Lithuania was the major power in eastern Europe, and he is remembered as a peaceful and successful ruler. He also helped perpetuate the reign of the Jagiellon dynasty, one of the most renowned families of monarchs in Europe. His marriage to Elizabeth of Hapsburg in 1454 produced six sons and seven daughters. One son became king of Bohemia and Hungary, and three others succeeded him on the thrones of Lithuania and Poland. Five daughters married German princes. (See also Jagiellon Dynasty.)